This invention relates to molding of resin compounds and, more particularly, to molding apparatus and methods for making parts with quality surface finishes.
High quality surface finishes have long been provided by stamping panels from metal sheets. Stamping dies cut through the metal sheets and contour the panel into the desired shape. The surface finishes provided by such stampings are of sufficiently high quality that they can be painted by conventional techniques to provide aesthetically pleasing panels free from surface irregularities. Such panels are typically used as exterior body panels in the automotive industry. While these stamped body panels initially are characterized by quality surfaces such surfaces can be degraded over time through corrosion, rust and the like. Perhaps an even more serious drawback is the weight that these metal panels add to the total vehicle weight thus degrading fuel efficiency and lending itself to other problems due to increased weight in the vehicle.
To overcome some of these problems, panels made from lighter weight plastic composite material have been proposed and actually used in some vehicles. These plastic panels are generally manufactured by compressing a resinous compound between two dies in a mold. The Budd Company, the assignee of the present invention, has been a leader in providing high quality plastic panels made from sheet molding compound (SMC). Briefly, SMC is a compound or charge in the form of sheet-like material containing a thermosetting resin, reinforcing fibers, fillers and the like. The SMC charge is placed in the mold and compressed between heated dies to polymerize the resin and form the final part.
While plastic composite panels provide significant advantages over their metal counterparts, great care must be taken to ensure that the plastic composite panels have the same high quality surface finish. Trapped air in the charge can create surface imperfections such as pits, porosity or other surface blemishes. These surface imperfections can often be corrected by post-molding filling and sanding operations. However, such operations add to the manufacturing costs for such parts.
Therefore, it is an object of this invention to provide techniques for molding plastic composite panels with high quality surface finishes that require a minimum of post-molding operations.
It is a further object of this invention to provide other improvements to increase production efficiencies not only by increasing the quality of the parts made by a given mold but also by designing the molding equipment so that it is robust and reliable while at the same time minimizing its footprint, i.e., the space on the plant floor that the molding equipment must occupy.
These and other objects and advantages are provided by the vacuum assisted molding equipment and methods which will be described herein. In accordance with the teachings of this invention, molding equipment is designed so that it can create a significant vacuum in the mold cavity in a very short period of time and do so on a repeatable basis required under high volume production standards. Special attention has been given to the design of the sealing devices that seal off the molding cavity during each part cycle. The mold has also been designed to efficiently use plant floor space by optimizing the configuration of the molding equipment, especially the sealing devices.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.